Mounting choke



May 11, 1954 BLITZ 2,678,392

MOUNTING CHOKE Filed June 12, 1952 F'IG. 2

A v INVENTOI? l. 2/ DAN/EL BLITZ ATTORNE'V Patented May 11,1954

MOUNTING CHOKE Daniel Blitz, Boston, Mass., assignor to RaytheonManufacturing Company, Newton, Mass, a corporation of DelawareApplication June 12, 1952, Serial No. 293,225

4 Claims.

This invention relates to chokes for the mounting of radio equipmentsuch as antennas to be mounted with their outer surfaces flush with aconductive sheet such as the skin of an aircraft.

In mounting antennas flush with the skin of aircraft, it is oftendesirable that current at radio frequencies flow between the antenna andthe skin so that it would not be desirable to insulate the antenna fromthe skin. Furthermore, even if insulation were introduced at this point,it would be likely to accumulate dirt and metallic particles. This wouldreduce the impedance of the joint at radio frequencies. If an attempt ismade to connect the antenna to the skin directly, the vibration of theaircraft is likely to cause the contact of the antenna with the skin tobecome intermittent. This has the effect of amplitude modulating thecurrent flowing between the antenna and the skin to generate spurioussig nals which can interfere with the performance of the equipment. Apermanent fastening, such as riveting, can minimize this difficulty butthen the antenna is not easily removed for servicing.

These difliculties are overcome while still maintaining anaerodynamically smooth outer surface by the use of the mounting choke ofthis invention. This mounting choke forms with the antenna a shortedhalf wave transmission line at the operating frequency with its open endat the outer surface where it presents a virtual short 9 circuit to theantenna current. The mounting, which is in the form of a bracket, islocated in this transmission line at a point approximately a quarterwave away from the shorted end at the operating frequency, so that thereis a minimum flow of current at the point where the bracket is attachedto the antenna frame, so that the resistance through the contact pointis not critical. Thus, the mounting of the invention forms a choke atthe operating frequency. Conductive material collected at the jointbetween the antenna and the skin has no effect as there is already ashort circuit at this point. Any looseness or vibration at the fasteningpoint has no effect as good contact is not required here. In fact, aninsulating gasket can be inserted between the bracket and the frame. Thegap between the skin and the antenna may be filled with a waterproofresilient dielectric, such as cork, to prevent accumulation ofconductive material at the point of attachment.

The foregoing and other advantages, objects, and features of theinvention will be better understood from the following description takenin conjunction with the accompanying drawings, wherein:

Fig. 1 is a bottom view of a pair of antennas mounted in a conductivesheet with a choke structure of the invention;

Fig. 2 is a section taken along the line 2-"? of Fig. 1; and

Fig. 3 is a detail of a lower left corner of Fig. 2 showing a choke ofthe invention.

The reference numeral It designates the skin of an aircraft or othersheet of conductive material into which is fitted a frame it carrying apair of horn type antennas if that may be associated with the radioequipment of the aircraft. The horns l2 and associated equipment arearranged within a metallic case it that also serves as a shield for theequipment. This case It is attached to the frame it and to the skin it!by means of a mounting bracket it, best seen in Fig. 3. It can be seenfrom Fig. 2 that the bracket id is formed integral with the lower edgeof the case It. A flange 15 formed on the lower edge of the case 13 isattached to the skin it by rivets It or by other well-known means. Thebracket proper Ill is formed as a second flange coming out from the caseat a point located a distance equal to a half wave length at theoperating frequency upward from the outer surface of the skin iii. Thisbracket i l bends downward parallel to the case I3 for a distance equalto a quarter wave length at the same frequency and then turns inward toform a flange ii to which the frame I i is attached by a bolt and nut18. A gasket 2!) of insulating material is shown inserted between theflange ii and the frame it. This gasket is optional. In addition, astrip 2! is shown inserted into the joint for the purpose of keeping outmoisture and foreign substances that might lodge at the point betweenthe sides of the transmission line at a high impedance point, spoilingthe action of the choke by changing its transmission characteristics.This strip may bemade of cork or any other resilient waterproofinsulating material. The use of such a strip is not essential to theoperation of the invention, but is desirable. The physical length of thetransmission line must be reduced to maintain the same electrical lengthwith the insulating material introduced. When the radio equipment isoperating and the horns :2 are radiating or receiving radio frequencyenergy, currents at the operating frequency will flow in the frame I l,and it may be desirable, as pointed out above, that these currents alsoflow in the skin It]. It was pointed out above that the bracket It comesout from the case I3 a distance equal to a half wave length from theouter surface of the skin Ill, and turns downward to meet the frame II.The space between the case I3 on one side and the bracket 14 and theframe it on the other side forms a shorted half wave length section oftransmission line which offers a virtual short circuit to the currentsat the operating frequency and permits them to flow freely between theframe II and the skin Hi. It was pointed out above that the bracket is aquarter wave length long from the point where it comes out from the caseI3 to the point where it is attached to the frame II. This section ofthe structure forms a shorted quarter wave length section oftransmission line so that there is a minimum flow of current at theoperating fre-- quency through the junction of the flange ll and theframe ll. Thus the junction need not be a good electrical contact andany variation in its conductivity during use will not aifect theoperation of the equipment. For this reason, the gasket 2!] may be ofdielectric material, if desired. However, if foreign substances, such asmoisture or particles, are allowed to accumulate across the transmissionline between case it on one side and bracket I l and frame it on theother side, its electrical length is altered. This is prevented by thecork strip 2!. When the space between the case l3 and the bracket 14 andframe H is filled with cork, as shown in the drawing, the effective Wavelength of the operating frequency within the space is reduced and thedimensions of the parts of the mount are reduced over what they would bein the absence of such a strip. The electrical lengths of the parts ofthe mount need not be exactly the half and quarter wave lengthsspecified. It is sufficient if the dimensions approximate these optimumdimensions sufficiently closely to result in a relatively low impedancepath between the outer surface of the antenna frame and the skin of thecraft and a relatively high impedance at the point of attachment.

This invention is not limited to the particular details of construction,materials and processes described, as many equivalents will suggestthemselves to those skilled in the art. It is accordingly desired thatthe appended claims be given a broad interpretation commensurate withthe scope of the invention within the art.

What is claimed is:

1. A mounting for a radio frequency energy radiator that is to bemounted flush with the surface of a sheet of conductive materialcomprising a bracket having a J-shaped main portion with the long armapproximately half as long as a wave length at the operating frequencyof the energy to be radiated and the short arm approximately a quarteras long as a wave length of the operating frequency to provide a virtualshort between the conductive sheet and the outer surface of theradiator, and a low current flow at the point where the radiator isconnected to the bracket.

2. A mounting for a radio frequency energy radiator that is to bemounted flush with the surface of a sheet of conductive materialcomprising a bracket having a J-shaped main portion with the long armapproximately half as long as a wave length at the operating frequencyof the 4 energy to be radiated and the short arm approximately a quarteras long as a wave length at the operating frequency to provide a virtualshort between the conductive sheet and the outer surface of theradiator, a low current flow at the point where the radiator isconnected to the bracket, and resilient dielectric material insertedbetween the arms of the bracket.

8. In combination a radio frequency radiator, a sheet of conductivematerial and means to mount said radiator with its external surfaceflush with the surface of said conductive sheet comprising a brackethaving a J-shaped main portion with the long arm of a length equal toapproximately half a wave length at the operating frequency of theradiated energy and the short arm of a length equal to approximately aquarter of a wave length at the operating frequency with means to attachthe end of the long arm to the conductive sheet, mean to attach theshort arm of the bracket to the radiator at a point located a distanceequal to approximately a quarter wave length at the operating frequencyfrom the outer surface of the conductive sheet so that the arms of thebracket and the part of the radiator between the point of attachment ofthe bracket to the radiator and the outer surface of the radiator is ashorted half wave length section of transmission line at the operatingfrequency, thus presenting a short circuit to currents at the radiatedfrequency flowing between the outer surface of the radiator and theconductive sheet, and the point of attachment located at a point wherethere is a minimum of current seeking to flow between the radiator andthe mounting bracket.

4. In combination a radio frequency radiator, a sheet of conductivematerial and means to mount said radiator with its external surfaceflush with the surface of said conductive sheet comprising a brackethaving a J-shaped main portion with the long arm of a length equal toapproximately a wave length at the operating fre quency of the radiatedenergy and the short arm of a length equal to approximately a quarterwave length at the operating frequency with means to attach the end ofthe long arm to the conductive sheet, means to attach the short arm ofthe bracket to the radiator at a point located a distance equal toapproximately a quarter wave length at the operating frequency from theouter surface of the conductive sheet so that the arms of the bracketand the part of the radiator between the point of attachment of thebracket to the radiator and the outer surface of the radiator is ashorted half wave length section of transmission line at the operatingfrequency, thus presenting a short circuit to currents at the radiatedfrequency flowing between the outer surface of the radiator and theconductive sheet, and locating the point of attachment where there is aminimum of current seeking to flow between the radiator and the mountingbracket, and resilient dielectric material inserted between the arms ofthe bracket.

No references cited.

